ANASTOMOSING FLUVIAL SYSTEM OF THE CEDAR MOUNTAIN FORMATION, EASTERN UTAH: A PALEOENVIRONMENTAL AND TAPHONOMIC ANALYSIS

An anastomosing river system has been identified within the Ruby Ranch Member (RRM) of the Lower Cretaceous Cedar Mountain Formation (CMF) of Utah and western Colorado. By characterizing typical features of fluvial channel bodies in the RRM, we have recognized that a particularly anomalous, dinosaur bone-bearing sand body in the RRM developed as a waterhole, or channel intersection, in this anastomosing system.

Compilation and correlation of multiple stratigraphic sections, analysis of individual channel characteristics and overall fluvial architecture of the RRM, combined with flume experiments designed to simulate the behavior of RRM channels, allows detailed paleoenvironmental interpretation. The RRM is dominated by horizons of mudstones with extensive and pedogenic calcretes, but is punctuated by isolated, single-story sandy to pebbly channel bodies. Analysis of channels shows pronounced similarities in the behavior of geographically isolated channels, including channel widths of ~100 m, depths ~2 m, and D90 of 8-16 mm. Low sinuousity, downstream bifurcation, and extremely rare lateral accretion surfaces further characterize anabranching, rather than meandering, channels. Lateral migration of channels and channel incision were limited by the presence of indurated calcretes below channels and at channel margins.

A modern analog at Cooper Creek, Australia provides an explanation for a taphonomic enigma of the RRM. At Dinosaur National Monument, a bone-bearing sandstone bed has produced the first sauropod skull from the Early Cretaceous of North America, along with bones of several other dinosaur taxa. The sandstone fill of the bone bed is unusual in the RRM. It is ~4 m thick, and exhibits 0.5-1 m flights of climbing ripples, whereas trough-cross bedding is the typical fill of RRM channel bodies. In flume experiments and in the modern behavior of Cooper Creek, deep scour (waterhole) occurs at the intersection of anabranching channels. We suggest that a deep scour or waterhole formed in the RRM fluvial system, and was filled rapidly during a flooding event, enabling rapid burial and preservation of fossil material.

Data for this study was gathered during the summers of 2002-2004 by 24 participants in the NSF-sponsored “Reconstructing Rivers” Research Experience for Undergraduates.